Method and device for welding strips of a thermoplastic material

ABSTRACT

A process and an apparatus for welding straps of thermoplastic plastics, wherein the supply of heat which is necessary for welding is controlled depending on the thickness at the weld of the straps which are to be welded to each other and is not switched off until said thickness falls below a predetermined set value.

FIELD OF THE INVENTION

This invention relates to a process and an apparatus for welding strapsmade of thermoplastic plastics, particularly packaging straps made ofpolypropylene or the like, which are heated over a limited weldingregion on their mutually facing faces and are pressed on to each otherunder the action of pressure.

BACKGROUND OF THE INVENTION

During the welding of straps made of plastics it is customary to supplyor to generate welding heat for a predetermined period of time whichexperience has shown is sufficient to soften the straps sufficiently inthe weld region and intimately to fuse them to each other on theapplication of pressure. It has been shown in this respect, however,that the strength of the weld is not always satisfactory, and thatwelding has not been performed correctly. The reason for this is basedon environmental effects and on irregularities in the supply of heat,which occur, for example, if there is a drop in the voltage of anelectrical source of heat and thus in the temperature also. Particularlywhen employing friction welding devices which are operated by anaccumulator independently of the mains, the number of revolutions alsobecomes less as the voltage decreases, so that the oscillating device,which moves the superimposed straps towards each other in order togenerate frictional heat, moves more slowly. The resulting welding timeis then no longer sufficient to generate the requisite welding heat, andintimate fusion of the straps in the weld region no longer occurs.

SUMMARY OF THE INVENTION

The object of the invention is to avoid these disadvantages and toprovide a process and an apparatus with which plastics straps can bewelded to each other in a simple and reliable manner, even underchanging environmental effects. This object is achieved by the inventionin that in order to ensure correct welding the change in the totalthickness of the straps which are welded to each other is determinedduring the welding operation and is compared with a predetermined setvalue.

The process according to the invention has the advantage that thestrength of each weld is tested, since in fact the total thickness ofthe straps which are welded to each other at the weld location dependson the softening of the plastics and the intimate fusion thereof, and isthus at the same time a measure of the strength of the welded joint.Depending on a predetermined total thickness, the supply of heat to thestraps which are welded to each other is not interrupted untilsatisfactory fusion of the straps in the welding region is achieved.

Since the duration of the supply of welding heat no longer plays a part,perfect welding operations can be carried out using accumulator-operateddevices, even if the voltage of the accumulator has already dropped,since the lower temperature is compensated for by a longer time ofaction.

An apparatus for welding straps made of thermoplastic plastics,particularly of packaging straps made of polypropylene or the like,which is provided with a heating device for heating mutually facingfaces of the straps which are to be joined to each other, and which isprovided with a pressure element which can be moved at the weldapproximately perpendicularly to the weld faces in order to compress thestraps which are superimposed on a supporting surface, is characterisedaccording to the invention in that the heating device comprises aswitch-off device, which is operated directly or indirectly by thepressure element as soon as the thickness of the weld, and thus thedistance between the pressure element and the supporting surface, fallsbelow a predetermined set value.

A welding apparatus which is constructed in this manner does notautomatically switch off the supply of welding heat until sufficientheating and fusion of the straps is achieved at the weld, which ismanifested by a decrease in the total thickness of the superimposedstraps at the weld.

In the apparatus according to the invention, a signal transmitter can beassociated with the pressure element, which signal transmitterdetermines the changes in the distance between the pressure element andthe supporting surface and feeds corresponding signals to an evaluationelectronics unit, which generates a switch-off signal in order to switchoff the heating device when the distance between the pressure elementand the supporting surface reaches a predetermined set value. Sensorscan be used here which determine the changes in distance in acontactless manner, e.g. by an optical route, electromagnetically, or byultrasound.

In a mechanically operated welding apparatus, the pressure element canbe connected to a sensing element which senses the surface of thesuperimposed unwelded straps and which comprises an operating elementfor switching off the heating device. The pressure element here isdisplaceable to a limited extent in relation to the sensing element in adirection towards the supporting surface, and during its displacementacts on the operating element, which switches off the heating device assoon as the weld has reached the predetermined thickness.

The operating element can be a lever which is swivel-mounted on thesensing element, which lever is acted upon by a driver disposed on thepressure element and which comprises an adjustable switching finger atone free end, which switching finger operates a first switch when thedisplacement of the pressure element in relation to the sensing elementreaches a predetermined value.

By adjusting the switching finger, the set value for the total thicknessat the weld of the straps to be welded to each other can be adjusted.

It is advantageous if the first switch comprises a normally closedcontact and is connected in series with a second switch which switcheson the heating device if said second switch is operated during theaction of pressure of the pressure element.

A particularly simple form of construction is achieved if the sensingelement comprises a driver pin, the free end of which fits into anelongated recess, which extends in an axial direction, in the pressureelement, in which recess it is held pressed by a spring element againstthe end face of the recess which faces the supporting surface, and inwhich recess it can move against the action of the spring element whenthe pressure element moves in relation to the sensing element in adirection towards the supporting surface during welding.

The welding heat can be generated at the weld location in various ways.In one preferred embodiment of the invention, the heating device can bea friction welding device, the oscillating lever of which moves apressure piece which detects the upper strap, and which is part of thepressure element and is pressed together with the latter in a directiontowards the supporting surface and is moved transversely to the plane ofthe strap by the oscillating lever and generates frictional heat betweenthe straps at the weld location.

In another embodiment, the heating device can comprise a heatingresistance wire which is disposed at the weld location between thestraps which are to be welded to each other, which heats the straps atthe weld and which is embedded in the straps when it is compressed bythe pressure element and remains there.

The process according to the invention, and preferred embodiments of theapparatus according to the invention, are explained in more detail withreference to examples of embodiments by the description given below andby the drawings, where:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an apparatus for welding and cutting offthermoplastic tightening straps made of plastics;

FIG. 2 is a plan view of the subject of FIG. 1;

FIG. 3 shows the subject of FIG. 2 as a vertical section on an enlargedscale along line III—III, with the inserted plastics straps before thecommencement of the welding operation;

FIG. 4 is an illustration analogous to that of FIG. 3, showing thewelding apparatus on the completion of the welding operation;

FIG. 5 shows the subject of FIG. 4 as a partial vertical section alongline V—V;

FIG. 6 shows the subject of FIG. 5 as a horizontal section along lineVI—VI; and

FIGS. 7 and 8 show another embodiment of the invention, as illustrationscorresponding to that of FIG. 5, before and after the welding operation.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, reference numeral 10 denotes an apparatus for weldingstraps II made of plastics, which is constructed as a manually operateddevice and which comprises a housing 12, a handle 13, a clamping lever14 and a operating lever 15. A laterally protruding supporting plate 16is situated on the underside of the housing 12, and enables theapparatus 10 to be placed on a packing unit 17 around which a strap I1made of plastics is to be looped.

The upper supporting surface 18 of the supporting plate 16 and thewelding apparatus 19 which is accommodated in the housing 12 form astrap channel 20, in which the strap ends 11 a and 11 b of the strap 11which is placed round the packing unit 17 are inserted so that they aresuperimposed on the supporting surface 18, which delimits a weldingregion or which delimits the weld location 21. The lower strap end 11 ais clamped to the supporting plate 16 by means of a swivelling clamp 22and the upper strap end 11 b is pulled through an opening 23 in theclamp 22 and is held tensioned in the direction of the arrow 24 by atensioning device, which is not illustrated, or by hand (FIG. 3).

When the strap ends 11 a and 11 b are inserted in the strap channel 20,the clamp 22 is lifted by raising the spring-loaded pressure bar 25,which is linked to the clamp 22, by means of the clamping lever 14.

The welding apparatus 19 by means of which the strap ends 11 a and 11 b,which are superimposed and tensioned on the supporting surface 18 of thesupporting plate 16, are welded to each other, is a friction weldingdevice with which the heat for welding the superimposed strap ends 11 aand 11 b is generated, by moving the strap ends 11 a and 11 b, which arefirmly pressed on to each another, to and fro in relation to each other.Friction welding devices of this type are known and are described andillustrated, for example, in German Patents No. 31 18 710, No. 28 02 034and No. 29 33 302. The friction welding device which is used in thepresent invention, which constitutes the heating device for generatingthe heat which is necessary for welding, will therefore not be describedin greater detail here, but reference is made instead to theaforementioned patent specifications, where the only parts of thefriction welding device which will be mentioned are those whichcooperate with other parts which are essential to the present invention.

The welding apparatus, which generates frictional heat and which thusconstitutes the heating device 19 in the sense of the invention,comprises a pressure piece 26 which is shown in FIGS. 3 to 6. Saidpressure piece rests on the upper strap end 11 b and its furrowedunderside is in positive engagement with the upper strap end 11 b. A peg28 which is fixed to the upper face of the pressure piece 26 fitsthrough an oscillating lever 29, which is moved to and fro in a rapidsequence, by an eccentric drive which is not illustrated, in thedirection of the arrow 30 transversely to the longitudinal direction ofthe strap 11. In the course of this procedure, the furrowed underside 27of the pressure piece 26 entrains the upper strap end 11 b and rubs itto and fro on the lower strap end 11 a, whereby frictional heat isgenerated at the weld location 21. The frictional heat heats the strapends 11 a and 11 b over a limited welding region to such an extent thatthe plastics material of the strap ends melts on the mutually facingsurfaces thereof and the plastics mix with each other, whereby the strapends are welded to each other under the action of pressure.

The pressure which is necessary for generating frictional heat and forproducing the weld is generated by a pressure element 31, which isformed by a pressure plunger, which is vertically displaceable above thepressure piece 26 in a sliding guide 32. The pressure element 31 issubjected to the action of a powerful pressure spring 33, which in turnis supported at one end on an upper collar 34 of the pressure element 31and at its other end in a recess 35 in the aforementioned operatinglever. At its lower end 31 a, the pressure element 31 has a recess 36which is open at its edge and in which a pressure roller 37 isaccommodated via which the pressure element 31 acts on the upper endface 38 of the peg 28 whilst the latter is moved to and fro under thepressure roller 37 by the oscillating lever 29.

It can be seen that the pressure spring 33 is compressed and attempts topress the pressure element 31 downwards when the operating lever 15 isswivelled clockwise in the direction of the arrow 39. When the operatinglever 15 is released, pressure can no longer be exerted on the pressurepiece 26, but the latter remains resting on the upper strap end 11 b dueto its own weight. So that it can be raised, and so that the pressureelement 31 returns to an upper inoperative position in which theaforementioned strap channel 20 becomes free for the insertion andwithdrawal of the straps, two lifting springs 40 and 41 are provided,the upper lifting spring 40 of which is supported at one end on a ring42 which is positioned above the sliding guide 32 and at its other endagainst the underside of the upper collar 34, and the lower liftingspring 41 of which rests on the oscillating lever 49 and exerts pressurefrom below against a peg collar 43. The pressure element 31, thepressure roller 37 and the peg 28 with its pressure piece 26 are therebyheld in positive engagement even in their raised state.

It can be seen from FIGS. 3 and 4 that a second sliding guide 44, inwhich a sensing element 45 can slide parallel to the pressure element31, is provided at the side of and near to the first sliding guide 32for the pressure element. On its side facing the pressure element 31,the sensing element 45 has a driver pin 46, which fits through anelongated hole 48 disposed in the wall 47 between the sliding guides 32and 44 and the free end 46 a of which fits into an elongated recess 49which is disposed in the pressure element 31 and extends in an axialdirection 50. The free end 46 a of the driver pin 46 is under the actionof a small spring element 51, which holds the driver pin 46 pressedagainst the end face 52 of the elongated recess 49 which faces thesupporting surface 18.

Behind the sensing element 45 there is an operating element 53 in theform of an elbow lever which is swivel-mounted at the lower end 45 a ofthe sensing element and which bears a switching finger 54 at its upper,free end 53 b. The switching finger 54 is constructed as a grub screwwhich can be adjusted in a tapped hole 55 so that its bottom end 54 aprotrudes beyond the operating element 53 to a greater or lesser extent.

At its lower end 53 a, the operating element 53 has a laterallyprotruding limb 56, which is acted upon by a driver 57 disposed on thepressure element 31 so that the operating element swivels clockwiseabout its pivot 58 when the pressure element 31 is moved downwards inrelation to the sensing element 45.

A first switch 59 and a second switch 60, which are connected in series,are provided for switching the heating device 19 on and off, i.e. in thepresent case for switching the friction welding apparatus on and off.The first switch 59 has a normally closed contact, which normally holdsit in its closed position. However, the second switch 60 is normallyopen and is closed when the operating lever 15 is pressed down on to it.If both switches 59 and 60 are closed, the heating device 19, i.e. thefriction welding apparatus, is supplied with current and is thereby setin operation.

The apparatus shown in FIGS. 3 and 4 additionally comprises a cuttingdevice 61, with which the upper strap end 11 b is cut off during thewelding operation. The cutting device 61 consists of a cutter 62 whichis fixed to the lower end of a cutter bar 63 which is guided parallel tothe pressure element in a third sliding guide 64. When the clamp 22 israised, the cutter bar is entrained by the clamping lever 14 whichengages below the lifting pin 65. When the operating lever 15 swingsdownwards the cutter bar is pressed down by a pressure pin 66 which isdisposed in the operating lever 15.

The mode of operation of the apparatus is as follows:

After the strap 11 has been placed round the packing unit 17 and thestrap ends 11 a and 11 b have been inserted in the strap channel 20 withthe clamp 22 raised, and after the lower strap end 11 a has been clampedby releasing the clamping lever 14, the upper strap end 11 b istensioned by pulling it in the direction of the arrow 24. The apparatus10 is then in the position illustrated in FIG. 3, in which the pressurepiece 26 and the sensing element 45 are situated above the strap ends 11a and 11 b which are superimposed at the weld location 21. The firstswitch 59 of the switch-off device is closed and the second switch 60 ofthe switch-off device for the heating device 19 is open.

In order to weld the strap ends 11 a and 11 b to each other, theoperating lever 15 is pushed down. In the course of this procedure, itswivels clockwise and produces the following effects:

a) It compresses the pressure spring 33, which thereby overcomes thepressure of the lifting springs 40 and 41 and presses the pressureelement 31, and with it the sensing element 45, the pressure roller 37,the peg 28 and the pressure piece 26, against the surface 67 of thesuperimposed strap ends 11 a and 11 b;

b) it pushes the cutter bar 63 downwards with the pressure pin 66 viathe cutter spring 68, so that the cutting edge of the cutter 62 rests onthe upper strap end under spring pressure;

c) it presses on the switching pin 69 of the second switch 60 andthereby closes the circuit via which energy is supplied to the heatingdevice 19, namely to the friction welding apparatus. The drive which isnot illustrated but which is thereby set in motion then moves theoscillating lever 29 to and fro in the direction of the arrow 30, andthe oscillating lever thereby rubs the upper strap end 11 b in relationto the fixed lower strap end 11 a in the welding region at the weldlocation 21 until sufficient frictional heat is generated under thepressure of the pressure element 31 in order to melt the strap materialat the mutually facing surfaces thereof and to cause the straps to mergewith each other.

During this melting process, the pressure piece 26 which is attached tothe pressure element 31 is pressed lower, whilst the sensing element 45,which rests on the straps near the weld 21 in the unwelded region,maintains its original height.

When the pressure element 31 travels downwards, the free end 46 a of thedriver pin 46 slides upwards in the elongated recess 49 against theaction of the spring element 45, whilst the driver 57 which is fixed tothe pressure element 31 presses on the lower end 53 a of the operatingelement 53. The latter is thereby swivelled clockwise about the pivot 58and the switching finger 54 which is disposed at its free, upper end 53bpresses against the switching pin 70 of the first switch 59.

As soon as the distance d between the pressure piece 26, whichconstitutes the lowest point of the pressure element 31, and thesupporting surface 18, and thus the total thickness of the straps whichare welded to each other at the weld location 21, falls below apredetermined set value, the switching pin 70 is pressed down by theoperating lever 53 to such an extent that the normally closed contact ofthe first switch 59 opens. The drive of the friction welding apparatusis thereby interrupted, the oscillating lever 29 and the pressure piece26 come to a stop, and frictional heat is no longer generated, i.e. thesupply of heat to the welding apparatus is interrupted.

It can be seen that the optimum set value for the total thickness of thesuperimposed, welded straps at the weld can be adjusted by screwing inor unscrewing the switching finger 54 in the operating element 53.

FIGS. 7 and 8 show another embodiment of the apparatus according to theinvention, where parts which are identical to those of the previousembodiment are denoted by the same reference numerals. These Figuresshow the pressure element 31, the pressure roller 37, the peg 28 withits lifting spring 41, and the pressure piece 26, which is moved to andfro by the oscillating lever 29 and which generates frictional heatbetween the superimposed strap ends 11 a and 11 b whilst the pressureelement 31 provides the requisite frictional pressure via parts 37, 28and 26.

Just as in the previous example of an embodiment, the superimposed strapends 11 a and 11 b here are also compressed whilst they are heated andfused, so that the thickness d of the superimposed straps at the weldlocation 21 is less after welding than it is before welding. The changesin the distance between the pressure element 31 or between the pressurepiece 26 attached thereto and the supporting surface 18 are determinedby a sensor 71 comprising a signal transmitter, which suppliescorresponding signals to an evaluation electronics unit 72. The actualvalues are compared there with a predetermined set value and if theactual values and set values correspond a switch-off signal is generatedby the evaluation electronics unit which interrupts the power supply tothe drive of the oscillating lever 29, so that the pressure piece 26 isstopped and welding heat is no longer generated.

In FIGS. 7 and 8, the sensor 71 comprising the signal transmitter ismerely illustrated schematically above the pressure element. It canalso, of course, determine the downward movement of the pressure pieceitself or can sense the movement of another part which is attached tothe pressure element and which moves downwards therewith.

It can be seen that with the apparatuses described above it is possibleto ascertain whether welding has been carried out correctly bydetermining the change in the total thickness of the straps which arewelded to each other. The supply of welding heat is not interrupteduntil a predetermined total thickness of the straps to be welded to eachother has been reached, by generating a control signal in order toterminate the supply of welding heat or by operating a circuit breaker.

The invention is not restricted to the embodiments which have beendescribed and illustrated. Rather, numerous changes and additions arepossible without departing from the scope of the invention.

As has been mentioned above, the welding heat can also be generated inanother manner, for example by electrical resistance heating wires,which are introduced between the straps to be welded to each other andwhich remain at the weld after the welding operation is complete and arecut off with the upper strap end. It is also possible to insert a wiremesh between the straps and to heat it inductively, and to switch offthe induction current as soon as the weld the has reached the desiredthickness at which the strap surfaces have intimately fused to eachother.

What is claimed is:
 1. An apparatus for welding straps made ofthermoplastic plastics comprising: a heating device, a pressure elementmovable at the weld location, a supporting surface, said heating devicehaving a switch-off device, said switch-off device operated directly orindirectly by a pressure element when a distance between said pressureelement and said supporting surface falls below a predetermined setvalue, said pressure element being connected to a sensing element whichscans the surface of superimposed unwelded straps and which comprises anoperating element for switching off said heating device, said pressureelement being displaceable to a limited extent in relation to saidsensing element in a direction towards said supporting surface andduring displacement said pressure element acting on said operatingelement.
 2. An apparatus according to claim 1, wherein said operatingelement is a lever, said lever is swivel mounted on said sensingelement, said operating element is acted upon by a driver disposed onsaid pressure element and said operating element comprises an adjustableswitching finger at one free end, said switching finger operates a firstswitch when the displacement of said pressure element in relation tosaid sensing element reaches a predetermined set value.
 3. An apparatusaccording to claim 2, wherein said first switch comprises anormally-closed contact and is connected in series with a second switchwhich switches on said heating device if it is operated during theaction of pressure of said pressure element.
 4. An apparatus accordingto claim 1, wherein said heating device comprises a heating resistancewire which is disposed at said weld location between said straps whichare to be welded to each other, said heating device heats said straps atsaid weld location, and said heating resistance wire is embedded in saidstraps when said straps are compressed by said pressure element.
 5. Anapparatus for welding straps made of thermoplastic plastics comprising:a heating device, a pressure element movable at the weld location, asupporting surface, said heating device having a switch-off device, saidswitch-off device operated directly or indirectly by a pressure elementwhen a distance between said pressure element and said supportingsurface falls below a predetermined set value, said sensing elementcomprising a driver pin, said driver pin including a free end that fitsinto an elongated recess, said recess extending in an axial direction insaid pressure element, said driver pin being held in said recess pressedby a spring element against an end face of said recess that faces saidsupporting surface, said driver pin in said recess movable against theaction of said spring element when said pressure element moves inrelation to said sensing element in a direction towards said supportingsurface during welding.
 6. An apparatus for welding straps made ofthermoplastic plastics comprising: a heating device, a pressure elementmovable at the weld location, a supporting surface, said heating devicehaving a switch-off device, said switch-off device operated directly orindirectly by a pressure element when a distance between said pressureelement and said supporting surface falls below a predetermined setvalue, said heating device being a friction welding device, anoscillating lever being provided which moves a pressure piece and whichcontacts an upper strap, said pressure piece being part of said pressureelement and being pressed together with said pressure element in adirection towards said supporting surface and is moved transversely tothe plane of said straps by said oscillating lever and generatingfrictional heat between said straps at weld location.